This invention relates to coating compositions and, more particularly, to marine coating compositions having antifouling characteristics.
In the early history of shipping, a detrimental effect on the maneuverability and speed of the ships was noticed after they had been in operation for a period of time. This detrimental effect was found to be caused by the adherence of organisms on the ship's bottom, which phenomena is referred to in the nautical art as fouling. For many centuries, the adhering organisms were removed at the time of dry docking or through frequent careening.
During the 17th century, it was found that a copper sheathed hull escaped the effects of fouling for a considerable period of time. As a result of cost, weight and practicality in the use of copper sheathing, copper paints were formulated for nautical use which had an antifouling effect similar to that of copper sheathing. In addition to the antifouling copper ingredient, certain anticorrosive materials had to be added to these paints to prevent pitting and breakdown of the metal hulls.
The majority of the antifouling paints and coatings, which have been formulated as an improvement on the use of copper sheathing, have incorporated some kind of toxin. These toxic substances work by slowly migrating to the surface of the coating and subsequently forming a lethal concentration of toxin in the surrounding water. These toxins usually consist of heavy metals such as lead, mercury, arsenic, copper and tin as well as organic substances such as chlorinated hydrocarbons or fluorocarbons.
A major problem encountered with all toxic antifouling paints and coatings is the slow release of poisonous substances into the surrounding water. Although these coatings release the greatest amounts of toxins while the ship is in motion, the heaviest marine growth occurs when the ship is stationary. Since the broad spectrum toxicity of most of these substances is manifested at but a few parts per million, the environmental impact of these toxins being released into waters used for commercial fishing becomes apparent.
U.S. Pat. No. 3,575,123 (T. H. Shepherd and F. E. Gould, 1971) discloses the use of water-insoluble hydrophilic acrylate and methacrylate polymers as coatings for the underwater portion of ships and static marine structures. These hydrophilic polymers effectively reduced drag when applied as thin films. The major drawback of this material was its failure to retard the growth of marine fouling organisms which necessitated the incorporation of antifouling, toxic chemicals into the polymer. Although the leaching rate of the toxins from the water-insoluble hydrophilic polymeric compositions was slowed down somewhat, it appears that the inability of this coating to resist fouling in the absence of toxins showed that only a partial answer had been reached.